CPS vs. HEP - What's the Difference?

It's been more than 10 years since the Ch. VII of ASME B31.1 Power Piping Code was introduced to mandate Owners of thermal power plants to develop and use an O&M Program for Power Piping. Today, it's still common to hear "I just want a High Energy Piping (HEP) Program."; even from owners of new, large thermal plants.

While a search of the latest edition of B31.1 for "High Energy Piping" or "HEP" will not yield any hits, Nonmandatory Appendix V defines "Critical Piping Systems" to include piping that are part of the feedwater-steam circuit of a steam generating power plant and all systems which operate under two-phase flow conditions. So in a sense Critical Piping Systems is more broad than other industry definitions of the subset of plant piping whose reliability is of concern to plant owners/operators. Traditionally, HEP corresponds to (only) those large bore (NPS 4 or greater) piping systems that operate above 750F or 1025 psia.

[Update Oct 2020]: The recently released ASME B31.1-2020 revised Nonmandatory Appenxix V to clarify potential damage mechanisms that can affect CPS Piping. Paragraph V-12 previously addressed creep (only); in the 2020 Code release V-12 is now re-titled "Damage Mechanisms" and identifies the following mechanisms for consideration:

• Creep
• Fatigue
• Creep/Fatigue Interaction
• Corrosion Fatigue
• Differential Thermal Expansion/Contraction
• Thermal Fatigue
• Thermal Degradation
• Thermal Shock
• Thermal Ratcheting
• Erosion
• Flow-accelerated corrosion (FAC)
• Graphitization
• Corrosion
• Stress Corrosion Cracking
• Mechanical Damage (gouges, dents, etc.)
• Deformation/overload

Tetra Engineering has advocated this type of approach to degradation-specific piping monitoring and condition assessment for more than 10 years and are pleased to see that the ASME has adopted this in Nonmandatory Appendix V.

[End of Oct 2020 Edit]

For a modern CCGT, the B31.1 CPS Program will generally go beyond this definition to include:

• HP Steam
• HRH Steam
• CRH Steam
• LP Steam
• Feedwater
• Auxiliary Steam

and address the specific damage mechanisms that impact piping reliability for each of these systems. These assessments include turbine bypass piping as well as the main run piping.

Why didn't the ASME B31.1 Code for Power Piping consider CPS/HEP reliability? The short answer is that the B31.1 Code, until 2007, was exclusively a design code that evolved from the 1950s and was simplified in that only significant plant excursions were considered. That is, in general, only static conditions were to be considered in the design of the piping; i.e. did not conditions associated with plant transients. 

Chapter VII now addresses these transient conditions in addition to the other damage mechanisms. These mechanisms are those that are truly experienced in the CCGT plants of today and also even in Radiant plants. Other piping applications, such as Ch. III for Nuclear Class 1 piping have more comprehensive requirements.

HEP was a term that evolved in the mid-80s when regulated utilities developed a collective approach for evaluating the condition and expected remaining life of piping subject (primarily) to creep and fatigue. These industry initiatives followed a series of major piping failures. HEP programs of yesteryear were recognized as significantly improving power piping reliability after these tragic events.

The frequency of failures decreased after the '80s, but significant failures have occurred as recently as the spraywater supply line failure at the Iatan plant in Missouri in 2007; the year that Ch. VII was incorporated in B31.1 as a Mandatory Chapter. Iatan is still owned by Kansas City Power & Light and at the time of the failure participated in EPRI programs for HEP and Flow Accelerated Corrosion damage. The failure resulted in 2 fatalities to plant engineers. Recognize that EPRI is a member sponsored research organization; they are not a standard setting organization like ASME.

The ASME approach for Covered Piping Systems (CPS) is intended to address well-known damage such as creep, fatigue and corrosion while leaving the details of the O&M Program to the Owner with encouragement to address other mechanisms that challenge piping integrity. For CCGT plants, these other mechanisms include thermal quench failures, spray water supply piping failures and other damage which can result in personnel injury and/or major component failure and unreliability.

In this way, CPS includes what was done since the '80s with HEP - but more. For example, HEP programs typically focus only on HP Steam and Hot/Cold Reheat Steam piping (for thermal plants with reheat steam turbines.) CPS Programs reach beyond to tackle these additional risk areas such as attemperator piping quench failures.

The new O&M requirements in B31.1 were approved as it became clear that structural changes in the power industry; notably, the increasing number of merchant power plants lacked both personnel experienced in managing power piping degradation as well as corporate commitments to maintain piping reliability. Plus, the types of new thermal power plants was changing to gas-fired combustion turbine based combined cycle units.

CCGT plants have significantly different piping systems from coal-fired radiant plants and problems were experienced unexpectedly early in life with cracking and failures in Gr. 91 piping in HP steam and Hot Reheat steam systems as well as unanticipated damage in attemperator piping in turbine bypasses and HRSG inter-stage attemperators due primarily to significantly different dispatch (start/stops) than was assumed during plant design and other factors.

CPS Programs for CCGT today address a wider range of systems than HEP (for example, including feedwater, LP Steam and Aux Steam) as well as a broader set of degradation mechanisms, including:

• unanticipated thermal transients/conditions with special consideration for 2x1, 3x1
• creep
• degraded Gr. 91
• thermal quench from incompletely absorbed spray water in attemperators
• FAC - flow accelerated corrosion
• EC - erosion-corrosion
• CUI - corrosion under insulation
• high temperature steam drain cracking
• non-compliant material
• improper welding techniques, consumables and/or PWHT

Degradation-specific NDT inspection plans for each mechanism tailor the scope and frequency of inspection to each plant's unique configuration and design. Inspection plans for Gr. 91 piping are one of the highest priority activities under modern CPS Piping Programs. Cracks and creep voids are sometimes identified, especially in older CCGT; requiring repairs and sometimes replacement and/or design changes (for example, replacing a field welded laterolet with a forged tee.)

Pipe hanger walkdowns are a compulsory activity; essential to Condition Assessments required by B31.1 Ch. VII to assess the condition of the piping support system, especially for systems that operate in the creep range.

Ch. VII also requires that data from all of these inspection sources and design data be maintained in a recordkeeping system. Today, software systems are available to provide ready access to inspection records to address these requirements. Guidance is available in B31.1 Nonmandatory Appendix V which addresses some of the details.

CPS Programs typically address the scope and frequency of future NDT inspections as well as piping walkdowns needed to ensure reliable power piping. Specific re-inspection schedules are typically determined by a combination of factors, including: personnel risk factors (inspection locations that are near high traffic areas), weld characteristics (field weld vs. shop weld), materials (Gr. 91 or other CSEF), risk-based inspection, stress analysis and engineering experience,.

Whether you call your program CPS or HEP, it needs to address the mechanisms that are important to reliable operation over the life of your plant and it must comply with Ch. VII if your plant was designed after late 2007. For older designed plants, it's safe to say CPS Programs are a "best-practice."